The present invention relates generally to power plant block loading and, more particularly, to a method and system for coordinating block loading of a combined-cycle power plant.
Block loading is a method of loading a power plant after a partial or complete electrical blackout of the electrical grid (hereinafter “grid”). Here, the normal process of loading the power plant is not possible. When a grid is de-energized or broken up into isolated sections, the active generating unit must provide the frequency/speed control. A strategy of reconnecting load in small amounts or “blocks” may be employed to ensure that the operational limits of the power plant are not exceeded. The sudden reconnection of a large block of load to the grid may drag down the system frequency or cause the active generating unit to exceed operational limits and trip off-line.
An electrical power system generally consists of a grid, electrical loads, transmission lines, and power generating equipment, such as, but not limiting of, gas and steam turbines. In an electrical power system the amount of electrical load should be balanced by an equal amount of electrical generation. Any change in load should be matched by an equal change in generation. As a protective measure, all electrical loads and generating equipment are isolated from the transmission system after an electrical blackout. Reestablishing power to the system requires careful coordination of adding system loads and generation such that operation limits of the generating equipment are not exceeded.
The general process for restoring power after an electrical power system blackout may include: 1) Isolate all power generating equipment by opening generator or line breakers at each power station. 2) Isolate the transmission system from all electric loads by opening various transmission line breakers and/or local distribution system breakers. 3) Restart a power generating unit and prepare the unit to accept load. This power station must have “black start” capability, which is the ability to start without connection to the electrical grid. 4) Close the generator breaker at the black start power station. This is known as a “dead bus” closure. 5) Energize the transmission system to nominal system voltage. 6) Connect a load block to the electrical grid by closing a local distribution substation breaker. 7) The black start power station will respond to the load demand and increase power output to match the block of load. The generating unit must be capable of increasing load to the amount equal to the load block within seconds without exceeding any operating limitations. 8) Once generation conditions have stabilized, this process may be repeated and additional segments of the local distribution system brought on line. Part of the load blocks may be used to start other power plants that do not have black start capability.
In a combined cycle power plant comprised of at least one gas turbine and at least one steam turbine, at least one gas turbine is generally used to control total power plant output by changing fuel flow in accordance with a droop governor. The steam turbine output typically follows gas turbine output as additional steam is generated from the at least one gas turbine exhaust. The steam turbine does not provide the primary response to load demand. In applying block loads to a typical combined cycle power plant the gas turbine governor would increase fuel flow as a load block is connected to the grid. Then as steam production and steam turbine output increases the gas turbine unloads to maintain total plant output at a constant level to match electrical system load.
There are a few drawbacks with the currently known methods of blocking loading. The thermal transients due to a sudden loading and subsequent unloading of the gas turbine that would occur during block loading are not desirable. Block loading can also be inhibited by other operating limitations such as, but not limiting of, combustion stability, cooling system requirements, or the like. Depending on the size of a block load power plant operating limits may be exceeded. This may result in protective actions, such as, but not limiting of aborting the attempt to restore the grid.
For the foregoing reasons, there is a need for a method and system to coordinate the block loading of a combined cycle power plant such that operating limits are not violated. The method and system should operate the power plant in such a manner to remain well within operating limits, and improve the success rate of grid restoration.